Unlike an existing computerized tomography (CT) apparatus, a cone-beam computerized tomography system takes images while moving to a predefined angle about an object so that the three-dimensional (3D) and four-dimensional (4D) imaging of an object to be examined is possible, thereby obtaining three-dimensional anatomical information. Further, unlike MRI, the cone-beam CT system more easily takes images of hard tissue, such as bone, rather than soft tissue, such as muscular tissue. The cone-beam computerized tomography system is variously applied in medical treatment or convalescence as well as in the diagnosis of diseases.
However, since the X-ray beams of the cone-beam computerized tomography system irradiate a wide area, photons of the X-ray beams that have a reduced energy of 100 keV or less are scattered by the object to be examined and so a low spatial frequency is obtained. Therefore, when the X-ray beam reaches a sensing device, due to not being able to distinguish the scattered X-ray beam from the primary X-ray beam in the original state, an image which is partially indefinite with considerably inferior quality is obtained as a result.
The photographing of images is accompanied by phenomena such as degradation of contrast, increase in noise and inaccuracy of computerized tomography values. Hence, a process of analyzing and correcting the degree of scatter (scatter correction) is required in order to obtain a definite image.
Among several methods for scatter correction, beam-stop array method is known as an effective method. A beam-stop array partially intercepts scattered beams, so that it is possible to estimate and correct the distribution of the scattered beams. However, several problems still exist in the beam-stop array.
Particularly, there is a problem in that conventionally, additional scans are required to correct the distribution of scattered beams, and therefore, increasing the amount of scattered beams absorbed in an object to be examined. In order to overcome this problem, Korean Patent Publication No. 10-2012-0138451 discloses a system and apparatus for circularly scanning an object to be examined using a scatter image correcting device, performing one-dimensional scatter correction by detecting X-ray projection image data using a multi-line X-ray detector, obtaining a scatter correction projection image by removing estimation scatter image data obtained through the one-dimensional scatter correction, and restoring an image using a back-projection filtration method based on the obtained scatter correction projection image.
However, although the number of additional scans can be reduced using the system and apparatus, generally, a penumbra is produced at the edges of strip lattices that intercept the beams. In this case, there still exist problems difficult to overcome using the back-projection filtration method provided in the invention described above.
When radiation acts on an object, the penumbra is differentiated from the isodose area which evenly receives a certain amount of radioactivity, whereas the penumbra means an area that receives only 20% to 80% of the radioactivity. Therefore, valid data that may be obtained from the empty space portion is contaminated. As a result, there remains a problem in that an indefinite image is obtained. For this problem, the invention provides a method of forming the width of the strip lattices so as to be narrower than that of the empty space portions. However, there is a limitation in suppressing the formation of a penumbra by forming the width of the strip lattices to be narrow. Particularly, when photographing is performed throughout a wide area such as the chest and abdomen of a person, a penumbra still forms, and therefore, loss of image information occurs in the area of the penumbra. As a result, the image obtained differs from the actual object.
U.S. Pat. No. 7,486,773 discloses a method of estimating the degree of scatter of beams irradiated using a plurality of cone-shaped members rather than strip lattices. As such, the use of a plurality of members can reduce error during the process of estimating the degree of scatter of beams even when photographing is performed throughout a wide area. However, the degree of scatter is estimated using at least four images obtained by photographing four times. Therefore, an increase in the amount of radioactivity absorbed in the object to be examined may occur, along with an increase in the time required to photograph.